As hardware technology for image processing has been developed and user needs for image shooting have increased, various functions such as auto-focusing (AF) and optical image stabilization (OIS) have been applied to a camera module mounted to a mobile terminal such as a mobile phone and a smart phone as well as an independent camera device.
The auto-focusing function adjusts a focal distance to a subject by linearly moving a lens or an assembly having the lens in an optical axis direction so that a clear image is generated on an image sensor (CMOS, CCD or the like) at a rear end of the lens.
The auto-focusing function may be performed in various ways. Representatively, a magnet (a permanent magnet) is installed at an AF carrier (or, a mover), a coil is installed at a stator (a housing, or another-type carrier), and an electromagnetic force is generated between the coil (provided at the stator) and the magnet (provided at the mover) by a power applied with an appropriate magnitude and direction, thereby moving the mover in an optical axis direction.
In addition, in recent years, a device or an actuator in which the AF function and the OIS functions are integrated has been used. In this case, a structure for moving the OIS carrier (or, a frame, a lens assembly or the like) at which the lens is loaded in a direction perpendicular to the optical axis direction at the inside of the AF carrier is integrally implemented with the AF structure described above. In some embodiments, it is also possible that a lens is mounted at the AF carrier and an OIS carrier provided out of the AF carrier moves in a direction perpendicular to the optical axis direction.
Meanwhile, a conventional device having only an AF function alone or having the AF function and the OIS function together includes a magnet 520 for receiving a driving force by an electromagnetic force of an AF coil, as shown in FIG. 1, and balls 510, 510-1, 510-2 arranged in the same direction as the optical axis are interposed between the AF carrier (or, the mover) 500 and the housing (or, the stator) (not shown) to improve the behavior characteristics of the AF carrier 500 moving in the optical axis direction.
In this structure, a suitable distance may be maintained between the mover and the stator, and the AF carrier may be more flexibly and accurately moved in the optical axis direction by minimizing a frictional force by means of ball rolling, movement, and point contact with the ball.
In the existing technique, it is common to use a plurality of balls b1 to b6 having the same size (or, diameter) d1 to d6.
In this case, all the balls theoretically make the point contact at the same time, and thus it may be regarded that the horizontal direction of the AF carrier is maintained even though the AF carrier (or, the mover) 500 moves in the optical axis direction. However, this is different from the actual situation, and a defect may occur in the horizontal tilt of the AF carrier.
A representative reason for this is that all the balls cannot have a perfectly identical size and thus ideal sameness cannot be achieved, and thus the AF carrier 500 is not possible to make the point contact at the same time.
In addition, since the AF carrier 500 is not fixed at a specific position but repeatedly moves and stops in the optical axis direction, a static frictional force and a kinetic frictional force are generated with different intensities, and a clearance is generated due to the different frictional forces. For this reason, all the balls are not able to make the point contact at the same time, and thus a tilt defect occurs at the AF carrier.
Further, even though one side of the AF carrier is closely adhered to the ball due to an attracting force generated between the magnet of the AF carrier and the yoke provided at the stator, since the AF carrier has a shape extending in the horizontal direction, more gravity is influenced on a portion of the AF carrier farther from one side closely adhered to the ball, namely on a portion more extended. For this reason, all the balls are not able to make point contact at the same time. Since the above factors are applied in combination, a tilt defect occurs at the AF carrier.
Since a plurality of balls are simply disposed without consideration of the above problem in the existing technique, in the conventional device, when the AF carrier 500 moves in the optical axis direction, balls making point contact with the AF carrier 500 are frequently changed, which collapses the balance of the AF carrier 500. As a result, tilt defects θ1 and θ2 as shown in FIG. 2 occurs at the AF carrier 500.
The tilt defects deform an optical path, along which light is introduced to an image sensor 600 through the lens, as much as a maximum separation angle (θ=θ1+θ2). Thus, an error occurs in the focus adjustment as much, thereby causing a problem in generating a clear image.
In recent years, a camera module mounted at a smart phone or the like is realized with a light and slim design. If the camera module has a slim design as above, a ratio of width to thickness of the AF carrier is increased, and thus the tilt problem of the AF carrier as described above becomes more serious.
In addition, in the conventional device having the AF function, as shown in FIG. 1, three balls arranged in the optical axis direction are respectively provided at right and left sides. Here, if it is intended to miniaturize the camera module or the device having the AF function, the camera module should accommodate the volume of the balls, and thus there is a great need for a structure where the AF carrier is supported by a larger number of balls with a smaller diameter.